Plastic parisons or preforms are conventionally reheated and formed into hollow plastic articles by blow molding in a blow mold using high pressure fluid, with if desired using a stretch rod for stretch blow molding.
The preforms are preheated in ovens immediately prior to being transferred into a blow mold. Stretch rods may be incorporated with the blow mold and typically enter the preforms from its open end and extend before and during the blow stage to cause biaxial orientation of polymer molecular chains of the blown article.
A two stage reheat blowing machine of the type described in U.S. Pat. Nos. 4,793,960, 4,690,633 and 4,522,581 uses a series of pallets to carry preforms through a series of processing stages: thermal conditioning, stretch-blow molding and strip-off or ejection. At the stretch-blow molding station it is important that the preform is accurately aligned with the blow mold to insure that the finished blown article is symmetrically formed with respect to the neck finish.
The present invention deals with an apparatus for venting the blow mold during the foregoing procedure.
In most kinds of mold and casting processes a provision is made in the mold to allow air or gases to vent from the mold cavity so that the molding material can completely fill the cavity and form a part that is free from voids. Injection molding of plastic requires the venting to permit the escape of air while preventing the escape of the incoming plastic, which is being injected at high pressure, typically 20,000 psi. Vents in injection molds are therefore typically 0.001-0.003" wide. However, most plastic parts are made with constant wall thicknesses in the range of 0.010-080" and even for parts with large surface areas the volume of air in the mold to be vented is comparatively small.
The process of blow molding, including stretch blow molding, uses a preheated preform, or parison, which is enlarged rapidly by the admission of high pressure fluid, usually air at up to 1,000 psi, to the interior of the preform to inflate it until it is pressed against the interior walls of a closed blow mold cavity. Consequently, the comparatively large volume of air trapped inside the closed blow mold cavity before the preform is inflated must escape, or be vented, as rapidly as the preform is inflated. Typically, the blowing portion of the cycle is less than 1 second. In the case of a 2 liter beverage bottle the volume of air to be vented is slightly less than 2 liters. Although blow mold vents, typically in the parting surface of the mold, can be wider than injection molding vents, they are limited to 0.010" because wider vents cause an unsightly witness line to be formed on the outer surface of the article.
It is conventional to clamp mold halves together and to cut vents into the mold's parting surfaces to minimize these witness lines. Thus the clamping force is distributed over that area of parting surface that remains after vents have been cut. If this area is too small the mold halves will be deformed or "hobbed", a form of cold working, until the vents are eliminated and their area once again added to help support the clamp force. Meanwhile the elimination of the vents has made the mold useless.
U.S. Pat. No. 3,078,508 shows a blow mold made from porous material and provided with a vacuum-blow air pump. The air in the closed mold is evacuated before the preform is inflated to speed up the cycle and improve the part quality. However, a porous mold structure is inherently weak and the system is costly since a vacuum-blow pump and ancillary controls must be provided. U.S. Pat. No. 4,489,771 shows a venting device which provides a large opening to vent a large amount of gas while providing a means to automatically close the vent when the injected resin reaches it. The complicated device only operates at very local places on the parting line, and would not function if used in a blow mold, wherein there is no easily flowing resin to close the valve. Copending U.S. patent application Ser. No. 935,632 filed on Aug. 24, 1992, shows a toggle mechanism including a rotatable crankshaft and linkage means connecting the crankshaft to at least one mold portion wherein the overstroked position of the crankshaft forms a vent between mold portions.
It is therefore a principal objective of the present invention to provide a simple, convenient and easy to use apparatus whereby air trapped in a blow mold cavity is conveniently and expeditiously released without adverse effects on the blow molded product.